Ground-motion variability at the Highway 14 and I-5 interchange in the northern San Fernando Valley

Abstract The purpose of this article is to analyze possible causes for failure of the structures at the Highway 14 and I-5 interchange in the northern San Fernando Valley following the magnitude M 6.7 San Fernando earthquake on 9 February 1971 and the M 6.7 Northridge earthquake on 17 January 1994. This interchange consisted of a series of structures up to 600-m long; during the 1971 and 1994 earthquakes, several of these structures separated along expansion joints and collapsed. To analyze the possible causes of failure, we recorded aftershocks at and near the interchange for 2 weeks following the 1994 earthquake. The aftershock recordings reveal a substantial relative spectral amplification (in the 1- to 10-Hz range) of a factor of 3 to 6 for sites located at the interchange (which is situated on sandstone) relative to a nearby rock (gneiss) outcrop site. Sites within the interchange area have a variation in amplification of a factor of 2 to 4 in the 1- to 10-Hz range. Also, substantial secondary arrivals were generated at two locations in the interchange area. We attribute variations in ground motion at the interchange to be due to variations in the composition of the sandstone and to topographic effects. We computed the linear-response strong ground motion for three sites at the interchange from the 1971 and 1994 main events using aftershock recordings of the Northridge earthquake from three sites (the south and north abutments of the southbound connector of Hwy 14 to I-5 and a central location to the interchange) as empirical Green9s functions. The same source events are used for all three sites for a particular synthesis so that differences between synthesized strong ground motion are due to differences in location, geology, and topography of the three sites. Source rupture models are obtained from independent studies. We estimate (1) peak acceleration to have been near 1 g for both earthquakes, due to the topographic and geologic conditions and the geometry of the fault rupture; (2) differential displacement across the north and south extremes of the area from the Northridge earthquake to have been about 70 cm longitudinal and 40 cm transverse to the Hwy 14 to I-5 connector; (3) differential displacement across the north and south extremes of the area from the San Fernando earthquake to have been about 25 cm longitudinal and 20 cm transverse; and (4) relative motion to be fairly incoherent for frequencies above 0.5 Hz. These observations are attributed to local geologic and topographic conditions and to wave passage effects and could have contributed to collapse of structures during these earthquakes.

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